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Update cosign to v3.0.5 in Dockerfile #2316

Job	Run time
go / freebsd	2m 4s
project checks / commits, licenses...	1m 8s
build for / go 1.24	55s
build for / go canary	7s
go / darwin	2m 8s
build for / go 1.25	6m 23s
other / yaml \| shell	7s
go / linux (go canary)	8s
go / windows	2m 5s
go / linux	1m 56s
	17m 1s

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